Solid-state thermoelectric systems that can convert thermal energy to electrical energy directly are very attractive because they can be operated with various types of heat energy sources and they are environmentally benign. Furthermore, the systems can be operated in refrigeration, heat pump, or power generation mode, and they can be made either very large or miniaturized because of leg-type structures that are simpler than those of conventional systems that require hazardous working fluid or complicated moving parts. Consequently, thermoelectric systems are extremely promising for a variety of applications including, for example, cooling systems for the local heating in microprocessors and optical components, and power generation systems that recover waste heat from conventional power plants and in hybrid automobiles.
However, the conversion efficiencies for current thermoelectric materials based on bismuth telluride and/or semiconductors are not high enough to make them economically viable as compared to current energy conversion systems. In addition, popular thermoelectric materials tellurium and bismuth are toxic and hazardous. Thus, it has been impossible for current thermoelectric systems to become widely used.
With worldwide energy and climate crises on hand, research into high-Z thermoelectric materials has re-emerged with utmost importance. As energy prices rise and thermoelectric efficiency improves, the prospect of thermoelectric energy conversion devices becoming widespread is now on the horizon. In addition, heat transport from microprocessors is becoming more often the limiting factor in improving performance. Thermoelectric Peltier devices could play a significant role in upholding Moore's law in the near future.
Many recent studies on the improved properties of nanoscale thermoelectric materials have offered new options in engineering the thermoelectric properties S, σ, and κ, though few if any offer any fundamental understanding that may lead to bulk devices with ZT>5. While the present state-of-the-art, Bismuth Telluride, is well understood, materials such as strontium titanate with much higher known thermopowers offer both a non-toxic platform from which to engineer a higher ZT and a pathway to understanding thermoelectricity in more complex material systems.
As far back as 1967, a study on the transport properties of bulk strontium titanate (STO) and several of its doped forms revealed that oxygen-deficient strontium titanium oxide (STO) achieved a thermopower S as high as 800 mV/K. Other work has revealed that Lanthanum-doped STO (SLTO), such as having the formulae Sr1-xLaxTiO3, has a lower thermopower (˜300 mV/K at low La concentration, decreasing with added La) but much higher electrical conductivity σ and so can achieve higher ZT (˜0.1 at 300K). In addition, a recent study by H. Ohta et al. has brought yet new attention to the thermoelectric properties of STO, in which is attributed a “giant” thermopower to a 2DEG at a rutile-TiO2/STO interface, though the overall ZT of this device is low due to the low conductance of the interfacial layer compared with that of the surrounding substrate and thin-film.
U.S. Pat. Nos. 6,727,424 and 7,291,781 and U.S. patent Application Pub. Nos. 2006/0037638, 2003/0168641, and 2002/0037813 disclose certain complex oxides having certain thermoelectric properties.